Aqueous suspension including a mixture of at least one aqueous suspension of precipitated silica and of at least one latex

ABSTRACT

An aqueous suspension is provided including a mixture of at least one aqueous suspension of precipitated silica and of at least one latex, in which the aqueous suspension of precipitated silica has a solids content of between 10 and 40% by weight, has a viscosity lower than 4x10-2 Pa s at a shear of 50 s-1 and, after centrifuging at 7500 revolutions per minute for 30 minutes, produces a supernatant containing more than 50% of the weight of the silica initially in suspension.This suspension can be employed in compositions based on an inorganic binder and in concrete compositions.

The present invention relates to aqueous suspensions includingprecipitated silica and to their use in the preparation of compositionsbased on cement or on similar inorganic binder or compositions derivedtherefrom.

The invention applies to all types of compositions including aninorganic binder such as cement, a slag or the like, as base ingredientor in combination with other constituents.

For the purpose of the present description “cement composition” will beintended to mean a composition based on cement or any other similarinorganic binder and water. These compositions constitute products whichare used as they are, especially as coating, or else which are intendedto incorporate especially inorganic fillers of variable particle size.

Such compositions enclosing an inorganic binder and an inorganic fillerin granulate form are considered very generally as concretes.

Although the specialist customarily describes concrete more specificallyas a composition in which the inorganic filler is based on relativelycoarse granulates (of the order of 4 to 15 mm), and mortar as acomposition in which the inorganic filler is based on less coarsegranulates (smaller than 4 mm), the term “concrete” will be employed inthe present description without any distinction to denote all kinds ofcompositions, whatever their particle size, in order to simplify thedescription.

It is desirable to have the ability to control the properties of thesevarious compositions both during their manufacture and their use, aswell as the products derived therefrom after setting.

Thus, concrete compositions for large structures (production of walls,veils, posts, cement finishes, slabs, industrial floors) must satisfyindividual requirements with regard to:

rapid setting kinetics for early removal of shuttering;

low plastic shrinkage and absence of cracks particularly in the case ofcement finishes;

good mechanical properties at 28 days;

durability (resistance to abrasion, low permeability to gases and toliquids);

leakproofing in the particular case of the underground concretes ormarine concretes in contact with water.

In addition, the concretes may either be manufactured on the work siteor manufactured in a central plant and transported to the site of use(concrete ready for use). In order that the composition may retainacceptable properties, even for a short period, its stability in thehydrated state must be ensured, especially by preserving its fluidityintact and by limiting the segregation of the materials in suspension.

More specifically, architectural concretes, that is to say visibleconcretes, must additionally satisfy requirements where aesthetics areconcerned:

homogeneous surface appearance;

reduction of efflorescences which form surface salt deposits (inparticular in the case of acid-treated concretes).

Prefabricated concretes for the production of facade components, pavingstones, slabs and pipes must more particularly have a low permeabilityto gases and to liquids for an optimum durability, as well as the sameaesthetic properties as the architectural concretes for the visiblecomponents.

The mortar or rendering compositions for the secondary work (finishingrenderings, smoothing renderings, facade renderings, adhesive mortars,spray renderings) must themselves also satisfy a certain number ofindividual requirements:

good water retention to avoid the loss of water by entry into the poroussubstrate or by preferential surface drying (risk of cracking and ofsurface powder formation);

good adhesiveness to the substrate;

resistance to abrasion and impact strength;

good fluidity combined with good water retention (especially in the caseof smoothing renderings), these contradictory properties being difficultto obtain together.

In general, during the utilization, critical parameters are the fluidityto ensure the processing, the degree of bleeding or the adhesiveness tothe substrate and water retention.

To satisfy these requirements, appropriate additives are usuallyincorporated into cement compositions or in compositions derivedtherefrom. However, it is not rare for an additive to produce, besidesthe positive effect for which it is employed, a detrimental effect onanother property. Thus, a plasticizing agent may improve the fluidity ofa composition, but the high contents sometimes necessary to reach thedesired result very clearly promote bleeding and diminish the settingkinetics and hence the acquisition of mechanical strength at an earlyage (fight against cracking, suitability for removal of shuttering,early commissioning, etc.).

Similarly, setting retarders which allow a composition to be conservedand transported for a certain time after its preparation also have avery marked effect on bleeding and on the setting kinetics.

Such difficulties appear in particular in the case of cement-basedcompositions and of self-levelling fluid concretes which, according toconventional specifications, must exhibit strict rheologicalcharacteristics in respect of settling (measured with the Abrams cone)or static or dynamic spreading.

The solutions which make it possible to obtain fluid concretes ormortars, and which are known so far, consist either in optimizing theparticle size curve by the introduction of fines or of ultrafines asdescribed in patent EP-A-0 184 386, this solution exhibiting, besidesthe awkwardness of making it necessary to handle large quantities ofpowder, the disadvantage of being highly sensitive to small variationsin the content or quality of the products employed, which in practicemakes it difficult to employ on a work site, or to add large quantitiesof plasticizers, retarders and optionally water-retaining agents tomaintain the handleability of the material for a sufficient period,which generally entails a delay in setting which impairs the efficiencyof the work site (delayed removal of shuttering or need for finishing inthe case of early shuttering removal) and a great sensitivity to smallchanges around the optimized composition.

In practice, one is often confronted with problems of prohibitivedecrease in the handleability in the event of metering error, ofsegregation of the mortar or of the concrete, of high bleeding, oflengthening of the setting time and of lowering of the final mechanicalproperties, in particular in the event of overdosing with water.

In fact, it is found very difficult to produce cement compositions orcompositions derived therefrom, which meet all the essentialrequirements that are desired with a view to an application.

From U.S. Pat. No. 5,149,370 and GB-2 212 489 it was known to improvethe bleeding and segregation properties of compositions by adding silicasols thereto. However, the properties of these compositions were stillinadequate.

The present invention proposes to meet this demand for improvedadditives which make it possible to combine a number of advantageousproperties from among those referred to above.

The aim of the invention is to provide a new additive for cementcompositions or compositions derived therefrom, making it possible todecrease the bleeding and segregation effects of the concretecompositions.

A further object of the invention is to provide an additive making itpossible to combine an improved fluidity, good water retention, even inthe case of high contents of water and/or of fluidizing or plasticizingagents, and to impart a good durability to the set product, especially areduction in the permeability to water and to gases, making it possibleto reduce the efflorescences.

To this end, a further object of the invention is an aqueous suspensionincluding a mixture of at least one aqueous suspension of precipitatedsilica and of at least one latex, in which the said aqueous suspensionof precipitated silica has a solids content of between 10 and 40% byweight, has a viscosity lower than 4×10⁻² Pa s at a shear of 50 s⁻¹,and, after centrifuging at 7500 revolutions per minute for 30 minutes,produces a supernatant containing more than 50 % of the weight of thesilica initially in suspension.

The first essential component of the mixture forming the aqueoussuspension which is the subject-matter of the invention is an aqueoussuspension of precipitated silica of high solids content, exhibiting alow viscosity and good stability with time.

Such suspensions are described, together with the process for theirmanufacture, in FR-A-2 722 185.

The solids content of the said suspension is preferably between 15 and35% by weight. The viscosity of the said suspension is advantageouslylower than 4×10⁻² Pa s at a shear of 50 s^(−1.)

These suspensions are very stable and their stability can be assessed byvirtue of a sedimentation test which consists in centrifuging the saidsuspension at 7500 rev/min for 30 minutes. The quantity of silicapresent in the supernatant obtained at the end of this centrifuging,measured after drying the supernatant at 160° C. until a constant weightof material is obtained, represents more than 50%, preferably more than60% of the weight of the silica present in the suspension.

The quantity of silica present in the supernatant obtained aftercentrifuging advantageously represents more than 70%, in particular morethan 90% of the weight of the silica in suspension.

Another essential feature of these suspensions concerns the particlesize of the silica particles in suspension.

In fact, besides their high viscosity, the concentrated silicasuspensions known so far exhibit the disadvantage of comprisinglarge-sized agglomerates in suspension, which give rise to sedimentationin the course of time.

The particle size distribution of the materials in suspension can bedefined by means of the median diameter D₅₀, which is the -particlediameter such that 50% of the population of particles in suspension havea smaller diameter.

Similarly, D₉₅ denotes the particle diameter such that 95% of thepopulation of particles in suspension have a smaller diameter.

Another characteristic value of the suspensions is the deagglomerationfactor F_(D). This factor, which is proportionally higher the more thesilica suspension is deagglomerated, is indicative of the proportion offines, that is to say of the proportion of particles smaller than 0.1 μmin size, which are not detected by a commonplace particle size analyser.

The particle size characteristics of the silica suspensions aredetermined by virtue of a particle size measurement performed on thesuspensions with the aid of a Sympatec particle size analyser.

F_(D) is measured by introducing into a particle size analyser a knownvolume V of suspension diluted so as to obtain a silica content of 4% byweight, and is equal to the ratio (10×V in ml/optical concentrationdetected by the particle size analyser).

The silica agglomerates present in these suspensions are of small size.

The particle size distribution of the agglomerates in suspension ispreferably such that their median diameter D₅₀ is smaller than 5 μm andthe deagglomeration factor F_(D) is higher than 3 ml.

Advantageously, the diameter D₅₀ is smaller than 2 μm, the factor F_(D)is greater than 13 ml and, in addition, the diameter D₉₅ is smaller than20 μm.

The second essential component of the mixture forming the aqueoussuspension which is the subject-matter of the invention is a latex,namely an aqueous suspension of particles of natural or synthetic resin.

The said particles are advantageously products of polymerization of atleast one monomer containing ethylenic unsaturation.

The latex itself is preferably produced by emulsion polymerization of atleast one monomer containing ethylenic unsaturation.

In particular, the monomer containing ethylenic unsaturation may beadvantageously selected from styrene, butadiene, acrylic acid,methacrylic acid, esters, preferably C₁-C₁₂, of acrylic or methacrylicacid, vinyl esters and mixtures thereof.

The latex which can be employed in accordance with the invention mayalso include particles of homopolymer or copolymer resin, an examplebeing a styrene-butadiene rubber latex.

The particle size of the latex can vary in accordance with the intendedapplication. In a first advantageous embodiment this size is from 0.1 to5 μm. Latices in which the particle size is from 0.1 to 0.3 μm may bementioned in particular, preferably styrene/utadiene latices, or elselatices in which the particle size is from 1 to 5 μm, preferablyacetate/versatate latices. In a second advantageous embodiment this sizeis at most 100 nm; these nanolatices may be of very diverse nature.

The abovementioned two components are used in combination in proportionswhich are appropriate for the desired application.

In general it is preferable that the aqueous suspension according to theinvention should include from 3 to 25 parts by weight of silica,expressed as dry weight, per 100 parts by weight of suspension, moreadvantageously from 5 to 20 parts by weight of silica per 100 parts ofsuspension.

It is preferable, furthermore, that the dry weight of the latex shouldrepresent the value of 5 to 50 parts per 100 parts by weight of aqueoussuspension according to the invention, advantageously from 10 to 40parts per 100 parts of suspension.

The aqueous suspensions according to the invention may be incorporatedinto cement pastes or more generally cement compositions based oninorganic binder and water, which they provide especially withresistance to segregation and water retention, in combination with anincrease in fluidity as a result of a synergistic effect between thesilica suspension and the latex.

The invention therefore also provides compositions based on an inorganicbinder and water, which are characterized in that they include anaqueous suspension as defined above.

The inorganic binder may be of any known type, especially cement ofPortland CPA H.P, CPA 55, CPJ 45, CPA CEM I, CPA CEM I PM, CPA CEM IPMES, CPJ CEM II, CPJ CEM II PM, CPJ CEM II PMES, CHF CEM II and CLK CEMII type, blast furnace slag or pozzolanic binders. The ratio of thewater to the binder is variable and depends above all on the desiredfluidity of the composition. This ratio may be in particular from 0.3 to2, preferably from 0.3 to 1.5.

In these compositions it is preferable that the silica should representfrom 0.3 to 5% by weight relative to the binder, expressed in relationto dry weight, preferably from 0.8 to 1.5%.

It is also advantageous that the dry weight of the latex shouldrepresent the value of 0.2 to 50% by weight relative to the binder,expressed in relation to dry weight, preferably from 3 to 30%.

To supplement the effect provided by the aqueous suspension according tothe invention, the compositions may additionally include at least oneplasticizing agent. This agent may be selected from the substancescommonly employed for this purpose in cement compositions. It willadvantageously be selected from lignosulphonates, casein,polynaphthalene, in particular alkali metal polynaphthalene sulphonates,melamines, polymelamines, formaldehyde derivatives, alkali metalpolyacrylates, alkali metal polycarboxyates and grafted polyethyleneoxides.

Such an agent may advantageously be employed in a proportion of 0.1 to10% by weight relative to the binder.

This agent may be introduced separately from the aqueous suspensionaccording to the invention or else simultaneously by means of asuspension including the silica suspension, the latex and the said agentat the same time.

Similarly, the compositions may additionally include at least onewater-retaining agent which may be selected from substances commonlyemployed for this purpose in cement-based compositions. It willadvantageously be selected from optionally modified polyvinyl alcohols,polyethylene glycols, polyoxyethylenes, acrylic polymers, especiallypolyacrylamides, polysaccharides of bacterial origin, like xanthan gum,guar gums, cationized guar gums, carob seed extracts, alginates,pectins, celluloses, cellulose ethers, especially carboxyalkylcelluloses, alkyl celluloses, hydroxyalkyl celluloses such asmethylhydroxypropyl celluloses, polyvinylpyrrolidone, sugars, especiallydextrose, ribose, corn starches, wheats, cationized or otherwise,lignites, leonhardites and derived products, alkali metal polyacrylatesand polystyrenesulphonates.

Such an agent may advantageously be employed in a proportion of 0.01 to10% by weight relative to the binder.

Furthermore, the compositions may also advantageously include a settingaccelerator such as aluminium sulphate, in a content of 0.01 to 3% byweight relative to the binder (expressed in relation to dry weight). Inthe case of aluminium sulphate this content is expressed as weight ofanhydrous aluminium sulphate.

This agent may be introduced separately from the aqueous suspensionaccording to the invention or else simultaneously by means of asuspension including the silica suspension, the latex and the said agentat the same time.

Reinforcing fibres enabling the resistance to cracking to be improvedwill also advantageously be employed in the compositions. These fibresare preferably selected from polyvinyl alcohol, polypropylene,polyethylene, steel, polyacrylonitrile, cellulose, carbon, kevlar,polyamide and polyester fibres.

The cement compositions based on an inorganic binder and water whichhave just been described can be employed as such or combined with othermaterials, especially to form concrete (or mortar) compositions. As hasbeen explained above, concrete is intended to mean the mixture of aninorganic binder, water and granulates of variable particle size,especially sand gravel mixes, sands and optionally fines, or evenultrafines.

In this regard a further object of the invention is concretecompositions including an aqueous suspension described above. Thesecompositions can be obtained by mixing the suspensions according to theinvention with an inorganic binder, granulates and optionally additionalwater, or else by mixing a cement composition described above withgranulates and optionally additional water.

Consequently another object of the invention is the use of the aqueoussuspensions or of the cement compositions described above in concretecompositions, in particular speciality concretes of the abovementionedtypes and secondary work coats.

Aqueous suspensions or cement compositions will advantageously beemployed which correspond to at least one of the preferredcharacteristics indicated above with regard to the relative contents ofsilica and latex, and optionally of plasticizing or water-retainingagents and, where appropriate, of cement.

The proportion of granulates in these concrete compositions may bechosen in a manner known per se within the usual ranges corresponding tothe desired type of concrete.

The invention finds a particular application in so-called specialityconcretes which must exhibit particular properties with regard tosurface appearance, mechanical strength, durability of the finishedproduct, fluidity in processing and low tendency to segregation.

It is possible, for example, to mention fluid concretes, concretes forthe manufacture of slabs (cement finishes and fluid mortars forfinishes), submarine concretes which must be easily pumpable andinjectable, facing concretes, concretes prefabricated by centrifuging orhot pressing, surface smoothing and finishing coats, especially forfloors, coloured concretes which must have a homogeneous surface withoutefflorescence and a relatively light base colour, as well as lightweightconcretes, concretes for industrial floors, leakproof concretes andcementing concretes for oil wells, which must have a low porosity, theproperties of all these concretes being seen to be improved by virtue ofthe invention.

More particularly, in the case of fluid concretes and cements, the highfluidity which is frequently desirable is obtained by virtue of theinvention while the ability of the compositions to retain water isimproved, and this makes it possible to facilitate the positioning andto reduce the problems of cracking due to a loss of water entailing aplastic shrinkage shortly after the positioning.

A further object of the invention is thus the use of the aqueoussuspensions described above as water-retaining agent in cementcompositions or concretes. This use, which makes it possible to reduceplastic shrinkage and, as a result, cracking, is particularlyadvantageous for cement finishes and smoothing coats.

The advantageous properties imparted to the cement-based compositionsand to the concrete compositions by the aqueous suspensions according tothe invention include more particularly the low tendency for thesegregation of the hydrated compositions, especially compositions with ahigh water content.

In this regard, a further object of the invention is the use of aqueoussuspensions as defined above, as antisegregation agent in cementcompositions based on an inorganic binder and water and in concretecompositions, especially in compositions where the water/cement, or moregenerally water/binder, weight ratio is from 0.3 to 2, preferably from0.3 to 1.5.

The invention is also very advantageous for the production ofprefabricated facing concretes or leakproof concretes, because it makesit possible to obtain better dispersion of the cement and betterhomogeneity. The surface appearance is thus improved in terms ofuniformity and durability of the set product by virtue of the limitedentry of water and of gases.

In the case of the facing products and of smoothing and finishing coats,this is accompanied by a lightening of the colour and an improvement insurface appearance, by better resistance to carbonate formation and alimitation in the appearance of efflorescences, as well as a reductionin the corrosion of the reinforcements.

Another subject-matter of the invention is the use of aqueoussuspensions described above as agent for reducing the permeability togases and to liquids in the compositions based on an inorganic binderand water and in concrete compositions.

The resistance to the entry of water is a decisive advantage for theleaktightness of structures situated in a region liable to flooding orwet or of structures intended to receive water (reservoirs, dams,aqueducts, swimming pools and water conduits).

Other advantages of the invention will appear more clearly in the lightof the examples which follow.

EXAMPLE OF PREPARATION

Preparation of a Suspension of Precipitated Silica

1. A cake C1 of precipitated silica is prepared as follows.

Into a stainless steel reactor fitted with a stirring system usingpropellers and with heating using a jacket are introduced:

346 litres of water,

7.5 kg of Na₂SO₄ (electrolyte),

587 litres of aqueous sodium silicate which has an SiO₂/Na₂O weightratio equal to 3.50 and a relative density at 20° C. equal to 1.133.

The silicate concentration (expressed as SiO₂) in the base stock is then85 g/l. The mixture is heated to 79° C. while being kept stirred. 386litres of dilute sulphuric acid with a relative density at 20° C. of1.050 are then introduced into the mixture until a pH value of 8(measured at the temperature of the mixture) is obtained. Thetemperature of the reaction mixture is 79° C. during the initial 25minutes and is then raised from 79° C. to 86° C. over 15 min, and thenmaintained at 86° C. until the end of the reaction.

Once the pH value of 8 has been reached, 82 litres of aqueous sodiumsilicate with an SiO₂/Na₂O weight ratio of 3.50 and a relative densityat 20° C. of 1.133 and 131 litres of acid of the type described aboveare introduced jointly into the reaction mixture, this simultaneousintroduction of acid and of silicate being carried out in such a waythat the pH of the reaction mixture is constantly equal to 8±0.1. Afterall the silicate has been introduced, the dilute acid continues to beintroduced for 9 min so as to bring the pH of the reaction mixture to avalue of 5.2. The introduction of acid is then stopped and the reactionslurry is kept stirred for an additional 5 min.

The total duration of the reaction is 118 min.

A slurry of precipitated silica is obtained which is filtered and washedby means of a filter press in such a way that a silica cake G1 isfinally recovered, whose loss on ignition is 78% (hence a solids contentof 22% by weight) and whose Na₂SO₄ content is 1% by weight.

2. 4 kg of the cake G1 (obtained by press filtration and exhibiting asolids content of 22% by weight and a Na₂SO₄ content of 1% by weight),preheated to 60° C., are introduced into a Cellier crumbler mixer.

Then, during the deflocculation of the cake, 13.1 ml of a solution ofsodium aluminate (which has an Al₂O₃ content of 22% by weight and a Na₂Ocontent of 18% by weight; relative density: 1.505) and 7.47 ml of asolution of sulphuric acid containing 80 g/l (relative density: 1.505)are introduced simultaneously so as to maintain the pH at a value of6.5.

The Al/SiO₂ weight ratio is approximately 2600 ppm.

Maturing is allowed to take place for 20 minutes while the mechanicaldeflocculation is continued.

The silica suspension G2 obtained is characterized by:

a viscosity of 0.06 Pa s (measured under a shear of 50 s⁻¹for 1 minute);

a particle size such that D₁₀=5 μm, D₅₀=19 μm, D₉₀=60 μm.

After one week's storage, the following are observed:

the formation, at the bottom of the storage container, of a sedimentwhich is excessively difficult, or even impossible, to redisperse;

an increase in the viscosity of the suspension: its viscosity is then0.45 Pa s (measured under a shear of 50 s⁻¹ for 1 minute).

3. The chamber of a Netzch LMEI mill is fed with 2 litres of suspensionG2 (taken after the crumbling), exhibiting a solids content of 22% byweight, at a flow rate of 5 litres/h; the filling ratio of the chamberwith alumina beads (diameter: 0.6-1 mm) is 75% and the speed of rotationof the shaft is 2000 rev/min.

At the end of this wet milling stage a suspension is obtained,characterized by:

a viscosity of 29 mPa s (measured under a shear of 50 s⁻¹ for 1 minute);

a particle size such that D₁₀=1.13 μm. D₅₀=4.1 μm, D₉₀=9.33 μm;

a solids content of 22% by weight;

a quantity of silica found in the supernatant of 63% (according to thetest for sedimentation under centrifuging, defined above).

4540 g of the silica suspension thus prepared are introduced into astirred reactor and the pH is adjusted to 9.5, with stirring, with theaid of concentrated sodium hydroxide solution, to stabilize the mixture.

5000 g of a styrene-butadiene rubber latex with a solids content of 50%,exhibiting a particle size of 0.1-0.2 μm, marketed by Rhône-Poulencunder the name SB 112, are then introduced with stirring at a flow rateof 100 g/min.

After half an hour's stirring the pH is readjusted to 9.5 and asuspension exhibiting a viscosity of 30 mPa s under a shear of 50 s⁻¹ isobtained.

This suspension is perfectly stable with regard to sedimentation formore than three months.

It contains approximately 10% by weight (dry) of silica and 26% byweight (dry) of latex.

Example of Application 1

Production of a Facing Concrete

Test 1

A concrete composition is prepared according to the following formula:

Granulates (5-15) 1000 kg Granulates 0-4 (sand) 800 kg HPR cement 425 kgPlasticizer (Rheobuild 2000 PF marketed 1.7 kg by MBT) (0.1% relative tothe weight of cement) Water 147 l

The granulates are introduced into a Hobart type planetary mixer and aremixed for one minute, then the cement is introduced and mixing iscontinued for 30 seconds.

Next, while the mixing is continued, the water and the plasticizer areintroduced, together with 42.5 kg (10% relative to the cement) of theaqueous suspension from the reference example.

The quantity of silica added corresponds to 1% of the weight of cementintroduced.

The water/cement weight ratio of this concrete composition is therefore0.41.

Test 2

A concrete composition is prepared in a similar manner, the initialwater content being increased so as to obtain a water/cement weightratio of 0.49, the quantity of all the other constituents being keptidentical.

Comparative Tests 1 and 2

Compositions similar to those of tests 1 and 2 are prepared, in whichthe introduction of the aqueous suspension from Example A is omitted butthe water/cement ratio is preserved.

The characteristics of these four compositions are evaluated by castinga concrete into identical leakproof moulds and by demoulding after threedays.

It is noted first of all that in tests 1 and 2 the fluidity of theconcrete is better and the positioning in the shuttering takes placebetter. Furthermore, in tests 1 and 2 no segregation of granulates orbleeding is observed, and this was hitherto incompatible with goodfluidity.

The characteristics of the set concretes are also evaluated. Thewhiteness of the product is measured by means of a Xenotest calorimeterand quantified by means of the chromatic coordinates L^(*), a^(*) andb^(*) in the CIE 1976 (L^(*), a^(*), b^(*)) system as defined by theInternational Commission on Illumination and listed in the Collection ofFrench Standards (AFNOR) colorimetric colour No. X08-12 (1983).

Better homogeneity of the surface and an appreciable lightening incolour are noted in tests 1 and 2.

Possible formation of efflorescence is verified after 28 days. At thistime no efflorescence has appeared in tests 1 and 2, whereas theproducts of the comparative tests exhibit numerous whitishefflorescences at the surface.

The results are recorded in Table 1 which follows.

TABLE 1 Silica suspension Efflor- (% relative Whiteness escence Test W/Cto cement) L a b at 28 days 1 0.41 10 58 −0.08 3.1  no 2 0.49 10   58.05−0.07 3.25 no comp. 1 0.41  0 55 −0.00 3.8  yes comp. 2 0.49  0 55 −0.033.99 yes

It would appear that the combined action of the silica and of the latexcontributes a filling of the porosity of the concrete, which sooner orlater reduces the efflorescences.

It is also observed that the water and gas permeability of the concretesof tests 1 and 2 is reduced in comparison with those of the comparativetests 1 and 2, which results in greater durability due to resistance tocarbonate formation, to attack by corrosive water, etc.

In addition, the adhesiveness of the concrete to a substrate is improvedas a result of the presence of the silica suspension and of the latex,as is the resistance to abrasion (decrease in dust formation).

A decrease in the number of cracks, due to an improvement in waterretention, is also observed.

Example of Application 2

Production of a Cement Composition of High Water Content (or grout)

This example aims to demonstrate the synergy effect produced by thesilica suspension and the latex on the rheological and stabilityproperties of cement pastes with a high water content.

A first series of tests not in accordance with the invention is carriedout, showing that the silica or the latex alone do not solve theproblems presented by these grouts, which are compared with two testsaccording to the invention.

Comparative Test 1

A grout is prepared including a silica suspension prepared in the sameway as in the example of preparation A but without adding latex. Itcorresponds to the following formula:

HTS cement 100 g plasticizer (Melmentplast N40) 2.5 g* setting retarder(Melretard) 0.8 g* aqueous silica suspension with a solids 5 g contentof 23% water 24 g *marketed by CIA

The water/cement ratio is therefore 0.28.

The water, the plasticizer and the retarder are introduced into a beakerstirred with a paddle and the cement is then added with stirring for 3minutes. The silica suspension is then introduced with stirring, whichis continued for 10 minutes.

Comparative Test 2

A similar grout in which the water/cement ratio is 0.35 is prepared inthe same way.

Comparative Test 3

A grout is prepared corresponding to the following formula:

HTS cement 100 g plasticizer (Melmentplast) 2.5 g retarder (Melretard)0.8 g styrene-butadiene latex as 50% aqueous 5 g suspension (SB 112)water 25 g

The water/cement ratio is therefore 0.28. The preparation is similar tothe comparative test 1, the latex being introduced instead and in placeof the silica suspension.

Test 1

According to the invention, a grout corresponding to the followingformula is prepared:

HTS cement 100 g plasticizer (Melmentplast) 2.5 g retarder (Melretard)0.8 g aqueous suspension from example A 9.6 g that is: silica (dryweight)   1 g latex (dry weight) 2.5 g water 22 g

The water/cement ratio is 0.28. The preparation is similar to that ofthe comparative examples.

Test 2

A grout similar to that of test 1 is prepared, in which the watercontent is adjusted to obtain a water/cement ratio of 0.35.

These five compositions are subjected to the following Theologicalmeasurements performed on a Rheomat 115 apparatus equipped with an MSDIN 145 cell. The grout is subjected to a shear increasing from 0 to1000 s⁻¹ over 1 minute and then to a constant shear of 1000 s⁻¹ for 1minute, and the shear is decreased to 0 s⁻¹ over 1 minute while theviscosity is measured. The final viscosity is recorded.

The tendency to segregation and to bleeding is evaluated:

by a static method: the appearance of a test tube filled with 200 ml ofgrout is observed over a period of 24 hours;

by a dynamic method under pressure: a given quantity of grout is placedin an API filter, above which an air pressure of 7×10⁵ Pa is applied;the quantity of water expelled from the paste is measured and, afterdrying at 160° C., the solids content of the upper part (air side) andof the lower part (filter side) of the filter cake obtained is measured.

The solids content of the lower or upper sample of the cake is definedby the formula:${{solids}\quad {content}} = {\frac{{{total}\quad {mass}\quad {of}\quad {the}\quad {sample}} - {{mass}\quad {of}\quad {water}\quad {from}\quad {the}\quad {sample}}}{{total}\quad {mass}\quad {of}\quad {the}\quad {sample}} \times 100}$

A difference between the two values indicates a heterogeneous cake, alower solids content higher than the upper solids content signifying asegregation.

The results are expressed by the following values:

the percentage of water collected by filtration${Wc} = {\frac{{quantity}\quad {of}\quad {water}\quad {collected}}{{initial}\quad {quantity}\quad {of}\quad {grout}} \times 100}$

 the water content of the cake after filtration${Cf} = {\frac{{{initial}\quad {water}} - {{water}\quad {collected}\quad {by}\quad {filtration}}}{{mass}\quad {of}\quad {the}\quad {filtered}\quad {cake}} \times 100}$

 relative to the initial water content of the grout Ci with${Ci} = {\frac{{initial}\quad {water}}{{mass}\quad {of}\quad {the}\quad {initial}\quad {cake}} \times 100}$

It is observed that, in relation to the comparative test 2, the combinedaddition of a silica suspension as defined in the invention and of alatex in test 2 results in bleeding properties which are superior tothose obtained with the silica suspension alone.

Furthermore, it is observed that the addition of latex alone in thecomparative test 3 results in a cement composition which has badbleeding and segregation properties.

Consequently, the excellent antibleeding and segregation properties ofthe additives according to the invention are wholly unexpected, insofaras a person skilled in the art would have expected a decrease in thebleeding and segregation properties on combining an aqueous silicasuspension with a latex, and not an improvement.

The results are collated in Table 2 below.

TABLE 2 Dynamic bleeding (retention of water under pressure) Upper Lowersolids solids Viscosity Static bleeding content content Test AdditiveW/C mPa s Bleeding Packing Wc % Cf % Ci % % % Comp. 1 Silica alone 0.28140  none none 7.1 15.7 22 83 84 (thixotropic behaviour) Comp. 2 Silicaalone 0.35 65 light yes 10.5 17.2 26 80.7 85.4 Comp. 3 Latex alone 0.2880 3.5 ml high 5.5 17.3 22 0 (*) 87 1 Silica + latex 0.28 93 none none9.5 13.7 22 86 86.4 2 Silica + latex 0.35 50 none none 12.5 15.3 26 8585 (*) Considerable separation resulting in filter blockage

From the above results it appears that the silica employed alone in thecomparative test 1 produces a homogeneous grout which is stable butwhich has a tendency to form a gel on standing (which is, however,destructured under low shear). Nevertheless, when the W/C ratioincreases, a tendency to bleeding and to packing (segregation) appears.In the dynamic bleeding test the cake obtained is more heterogeneous,which explains the packing observed in the static test. The silica is atthe limit of effectiveness.

In the comparative test 3, the tendency to bleeding and to segregationis very great in static conditions and under pressure. The latex has noadvantageous effect on these phenomena.

In test 1, a lesser tendency to gelling is noted in comparison with thecomparative test 1, the behaviour of the grout being quasi-Newtonian.The grout does not present any bleeding or segregation problem: itremains perfectly homogeneous in both static and dynamic conditions,although it is much more fluid than the grout in the comparative test 1.

In test 2, while the W/C ratio is 0.35, the grout remains perfectlystable and homogeneous in static and dynamic conditions, in contrast tothe grout of the comparative test 2.

Furthermore, although the grout is more fluid (50 mPa s against 65) amuch better water retention is observed in test 2.

What is claimed is:
 1. Aqueous suspension comprising a mixture of atleast one aqueous suspension of precipitated silica and of at least onelatex, wherein said aqueous suspension of precipitated silica has asolids content of between 10 and 40% by weight, has a viscosity lowerthan 4×10⁻² Pa s at a shear of 50 s⁻¹ and, after centrifuging at 7500revolutions per minute for 30 minutes, produces a supernatant containingmore than 50% of the weight of the silica initially in suspension,wherein said aqueous suspension is stable with regard to sedimentationfor at least three months without stirring.
 2. Suspension according toclaim 1, wherein the latex comprises a product of polymerization of atleast one monomer containing ethylenic unsaturation.
 3. Suspensionaccording to claim 1, wherein the latex has a particle size of 0.1 to 5μm.
 4. Suspension according to claim 1, wherein the latex has a particlesize of at most 100 nm.
 5. Suspension according to claim 1, comprisingfrom 3 to 25 parts by weight of silica, expressed as dry weight, per 100parts of suspension.
 6. Suspension according to claim 1, comprising from5 to 50 parts by weight of latex, expressed as dry weight, per 100 partsof suspension.
 7. Suspension according to claim 2, wherein the at leastone monomer containing ethylenic unsaturation is selected from the groupconsisting of styrene, butadiene, acrylic acid, methacrylic acid, estersof acrylic acid, esters of methacrylic acid, vinyl esters and mixturesthereof.
 8. A method for the preparation of a concrete or cementcomposition, comprising incorporating the aqueous suspension accordingto claim
 1. 9. The method according to claim 8, wherein said aqueoussuspension is an antisegregation agent in a cement composition.
 10. Themethod according to claim 8, wherein said aqueous suspension reduces thepermeability to gases and to liquids in a concrete or cementcomposition.
 11. The method according to claim 8, wherein said aqueoussuspension allows water retention in a concrete or cement composition.